Thanks for that Doug.
The mains waveform is detected by optocoupler U1. Input is at the bottom (on your drawing and on the photo) and output is on the top left pin, which is pulled up to the 5V VCC rail (which comes from the 5V regulator) by R2. This pin will normally sit at about half of VCC, or 2.5V. It feeds the IC that you've marked as U14, on pin 9.
U14 (or at least, the IC that the signal feeds into, at the top right of the board) is a 74C14 or CD40106 (actually it's both; they're different names for the same component). It's not a flip-flop; it's a hex inverter with Schmitt trigger inputs. It contains six inverter sections. Each inverter takes an input signal and produces an output signal that is the opposite logic state of its input, i.e. it inverts the signal. When the input is low, it will drive the output high, and vice versa.
One of these inverters has its input on pin 9 and its output on pin 8, which connects to the modular connector. This signal is the one the microcontroller uses to detect when the mains voltage crosses zero. It uses this timing information to control the conduction angle of the Crydom solid state relay, to vary the motor speed.
The voltages on pin 9 (inverter input) and 8 (inverter output) are 0.41V and 1.76V. This is not consistent. If the input measures 0.41V, then the output should measure 5-0.41V or 4.59V.
I think the 74C14 has been damaged, possibly by a voltage surge or electrostatic discharge (ESD), possibly involving the interconnect cable. I think if you replace it, you will fix the problem.